Viscous fingering in a yield stress fluid

We study the Saffman-Taylor or viscous fingering instability in yield stres s fluids. The theory for yield stress fluids shows that the dispersion equa tion of the instability is similar to that for Newtonian fluids; however, t he capillary number governing the instability now contains the yield stress . Experiments using gels and foams reveal very branched fingers in the gel. The results are in excellent agreement with theory for the gel, with, in a ddition, a crossover from yield stress dominated to viscous behavior. The r esults for foams are very different due to the existence of wall slip.